net: context: Refactor option setters

Set separate option setters for bool, uint8_t and uint16_t
values. Use those generic setters when storing the desired
option value.

The uint16_t option setter stores the value to uint16_t variable
and expects that user supplies int value.

For uint8_t value, it is expected that uint8_t value is supplied
instead of int.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>